It has been common for many years to perform seismic exploration for oil, gas and other minerals. Typical techniques employed involve generation of an acoustic wave at the surface of the earth, including the surface of the ocean. This wave travels downwardly into the earth and is reflected from subterranean layers of rock of interest upwardly towards the surface of the earth where its return may be detected. Typically the detectors output analog electrical signals which are converted to digital form and are recorded. Analysis of the recorded signals can then be performed and used to yield a picture of the subterranean structure of the earth which can be interpreted by geophysicists in the search for oil, gas and other valuable minerals.
As part of the expansion of this field, it is desirable to generate better and better pictures which in turn requires the recordation of more and more digital data. At the present time, a typical shipboard exploration system will involve the recordation of samples from 208 trailing geophones, each sampled every 4 msec. to 16 bit accuracy. Recordation of this data proceeds at such speed that an entire reel of magnetic tape is filled in on the order of 10 minutes with this data. Clearly it would be desirab1e to reduce the amount of data to be stored, if this could be accomplished without loss of accuracy.
A further impetus for the development of useful data compression schemes is that it is frequently desirable, for example, that the data be transmitted from an ocean-going exploration vessel to a home base computer for analysis, while the ocean-going vessel is still in the area of exploration. In this way, if the data is of particular interest, or if for some reason was improperly recorded requiring re-exploration, the ship can return to the area of exploration without having to sail an excessive distance. However, to transmit full digital representations of the seismic data recorded as mentioned above is prohibitively expensive using present day transmission facilities, a situation which is not likely to improve. This is therefore another area in which compression of seismic data could be of use.
Clearly, in order to be useful, data compression techniques may not be permitted to distort the signal too greatly upon decompression. Two particular factors are of concern; the RMS difference between the full word trace and the same trace after having been compressed and decompressed and second, the correlation of the difference trace with a trace of full word accuracy. The RMS error describes quantitatively how much distortion is present in the compressed and decompressed trace. The correlation indicates whether the errors made will stack out (that is, whether upon employment of prior art techniques to reduce random noise in seismic records by summing, the errors will likewise be reduced) and if not, what will be the pattern of the distortion.
Prior art techniques of data compression in seismic systems have failed to provide adequate results. These techniques have fallen into two basic categories. One class involves frequency domain techniques, such as shown best in Wood, L. C., "Seismic Data Compression Methods", Geophysics, V. 30 No. 4 (1974) pp. 499-525, and the other has time-domain techniques. Gain bit only quantization has been employed, that is, the full word is characterized by the exponent of the power of 2 nearest the sample; the exponent therefore only need be transmitted. Clearly, this may involve errors equal to half the difference between successive powers of 2. Moreover, the errors are correlated with the amplitude of the signal: as the amplitude of the signal rises, so too does the probable error. Two variations of differential pulse code modulation (DPCM) have also been tried, known as simple DPCM and running sum DPCM. These techniques in general attempt to use the fact that the difference between successive samples are smaller than the samples themselves. However, the running sum DPCM method, although very fast, results in rather high RMS noise. Simple DPCM has smaller distortion but requires a long low cut filter to eliminate propagation errors. Accordingly, while DPCM techniques are useful, there remains a need in the seismic art for improvement on the technique employed.
Application of DPCM techniques and speech encoding are discussed generally in N. S. Jayant, "Digital Coding of Speech Waveforms: PCM, DPCM and DM Quantizers" Proceedings of the IEEE, V. 62, No. 5, May, 1964, but in connection with speech waveform coding, not seismic data coding.
It will be appreciated by those skilled in the art that seismic data is of a generally repetitive character; that is, it involves a sinusoidal wave convolved with a reflectivity series. As such, the wave nature of the records, therefore, can be utilized in data compression and one need not assume that the data are random. It is this fact which is exploited by all the differential pulse code modulation techniques mentioned above. The advantage of differential pulse code modulation techniques over gain bit only coding or sign bit coding, in which a sign only is transmitted,
indicating whether a given sample is positive or negative, is threefold. First, the errors are many times smaller than those that occur when quantizing the full waveform into a fixed number of binary gain bits. Secondly, the errors are much less correlated than with either gain bit coding or sign bit coding, so that it is less likely that the errors will be enhanced when seismic data processing is performed thereon. Finally, differential pulse code modulation techniques are less sensitive to low frequency noise than are the gain bit and sign bit coding techniques and are therefore much more suitable for f-k filtering, a well known technique commonly employed in seismic data processing to reduce noise. Therefore, it is desirable to use some form of differential pulse code modulation, but as mentioned above, neither simple or running sum DPCM techniques yield results of low enough distortion to be useful.